The present invention relates to a sheet feeder and more particularly to a sheet feeder with a reversing drive mechanism.
The present invention is particularly adapted for use with automatic printing apparatus for producing prints of original documents as well as electronically created and digitally stored images which are made available through the use of electrostatographic printing techniques. Various types of sheet feeders have been used to feed copy or print sheets to the printing machine to obtain a toner image thereon or to feed documents which are to be reproduced onto the imaging platen of the reproduction machine. Thus, for example, devices rely on the operator manually aligning one edge of a document against the registration edge with the device subsequently automatically transporting the sheet in a direction parallel to the registration edge. These devices come in several forms including those wherein the registration edge is adjustable to accommodate sheets of different dimensions and also to enable the sheet feed mechanism to contact the top sheet in a stack of sheets in the center of the sheet. Alternatively, a side registration device may be employed wherein the registration edge is fixed relative to a fixed feeding mechanism. In both of these devices difficulties may be encountered with a feeding of sheets in that there may be a tendency for the sheet to skew in a direction lateral to rather than parallel to the sheet feeding direction. Sheet feeders such as friction retard devices with small foot prints demonstrate a particularly high sensitivity to sheet skew. For example, if there is a wall or something close to the edge of the paper as it is being fed that can contact the corner and buckle it a paper jam could result. In those devices where the feeding mechanism may not be in the center of the paper being fed, there is a natural tendency to generate a moment to rotate the paper in a counterclockwise direction (see FIG. 3A). If the corner of the paper will scrub against the registration edge which tends to turn it even more resulting in possible damage to the paper, creation of a buckle in the corner of the paper which eventually leads to a paper jam. In addition, the desire to feed many different sheet sizes with a fixed position feed head wherein the feed head is off center with the centerline of a paper can accentuate the problem causing in addition to sheet skew, centration problems in maintaining the sheet fed along the centerline. Furthermore, in many specific applications, the difficulties may be exaggerated because of the very close tolerance or small clearances permitted in the paper path. In these situations, a very, very small skew may cause the paper to contact a frame or other machine member buckling the corner and eventually resulting in a paper jam.
In several types of sheet feeders, nudger rolls are used to nudge the top sheet in a stack of sheets toward a feeder mechanism. Typically, this involves a rotatably driven roll on top of a stack of sheets which tends to shingle the stack and feed the top sheet to the main feeding mechanism. FIGS. 4 and 5A illustrate a typical geometry for such device wherein a pair of feed rolls 44 are rotatably driven by feed roll shaft 46. Nudger rolls 45 are pivotally supported about the feed roll on shaft 47 and an endless belt drive 50 couples rotational movement of the feed roll in the feeding direction to the nudger roll. A problem that frequently develops in such devices is that after the last sheet in a sheet feeding job has been fed the nudger rolls contact the feed table 40 and because of the direction of rotation in the feeding direction a torque is created by the rotation of the nudger rolls creating a normal force driving the nudger rolls onto the feed table. As a result, the nudger rolls are physically urged against the feed table and the operator has difficulty inserting a stack of sheets between the nudger roll and the plate to enable a subsequent feeding operation. Furthermore, in some devices, a switch is placed in the paper path to detect the impending insertion of a stack of sheets under the nudger rolls which activates the driving mechanism. This further complicates the situation in that in view of varying levels of operator skill and speed of insertion the nudger rolls and feed rolls could feed the top sheet to different positions. While this aspect of the operability problem can be corrected by removing the switch while activating the feed mechanism the difficulty created by not being able to insert a stack of sheets between the nudger roll and the feed table remains.